The major goal of the proposed research is to obtain information about the specific roles played by identified cell groups in the primate motor cortex in the voluntary control of forelimb muscle tension. Combined surgical and electrical stimulation techniques will be employed with anesthetized monkeys in order to investigate the dynamic muscle tension changes evoked by activation of corticospinal, corticorubral, and corticoreticular outflows from the arm area of the precentral gyrus. In addition, semi-chronic microelectrode techniques will be used to record from identified cells within each of these efferent cell groups, in conscious animals that have been trained to generate highly controlled and repeatable increases in the tension output of selected forearm muscles. From these experiments we hope to obtain quantitative information concerning the relative involvement of each of the various cortex cell populations in controlling (1) The relatively steady levels of muscular activity that are associated with supporting a fixed muscle load or joint position, and (2) The controlled, transient increases in muscular force that are associated with rapid, skilled movements.